UV photoreceptors and UV-yellow wing pigments in Heliconius butterflies allow a color signal to serve both mimicry and intraspecific communication

Mimetic wing coloration evolves in butterflies in the context of predator confusion. Unless butterfly eyes have adaptations for discriminating mimetic color variation, mimicry also carries a risk of confusion for the butterflies themselves. Heliconius butterfly eyes, which express recently duplicated UV opsins, have such an adaptation. To examine bird and butterfly color vision as sources of selection on butterfly coloration we studied yellow wing pigmentation in the tribe Heliconiini. We confirmed using reflectance and mass spectrometry that only Heliconius use 3-hydroxy-DL kynurenine (3-OHK) as a wing pigment. 3-OHK looks yellow to humans but it reflects both UV- and long-wavelength light whereas butterflies in related genera have chemically unknown yellow pigments mostly lacking UV-reflectance. Modeling of these color signals reveals that the two UV photoreceptors of Heliconius are better suited to separating 3-OHK from non-3-OHK spectra compared to the photoreceptors of related gene...